1. Field of the Invention
The present invention generally relates to a device and method for measuring alternating current magnetic susceptibility, more particularly to a device and method for measuring the alternating current magnetic susceptibility intensity variation and phase difference and/or the real part and the imaginary part of the alternating current magnetic susceptibility.
2. Description of the Prior Art
Within an external added field, the magnetic dipole moment direction of a magnetic particle is led to be along the direction of the external added field based on the magnetization effect. As the external added field is an AC (alternating current) magnet field with AC frequency not too high enough such as the low frequency area of a microwave frequency, the magnetic dipole moment direction is periodically oscillating there and back following the AC magnet field. At this moment, the oscillation frequency of the magnetic dipole moment is the same to the frequency of the external added field. However, the actual magnetic field direction does not match the expected magnetic field direction due to the instant magnetic dipole moment direction is uncertainly the same to the external added field direction. The periodically oscillating phase difference is so-called a magnetic field phase difference. Therefore, AC magnetic susceptibility of a magnetic particle within an AC magnetic field is able to be represented by the magnetic susceptibility intensity and the phase difference of the magnetic particle.
U.S. Pat. No. 6,683,452 provides a device for detecting magnetic characteristics. The device is used for detecting an internal crack of a metal or a shape of the metal, which comprises magnetic field generating unit, magnetic flux density measurement unit, a magnetic flux focusing unit and a display unit. The display unit further includes a synchronic portion, an amplification portion, a conversion portion and a display portion and used for real-time displaying changes in the magnetic flux density by analyzing the signals from the magnetic field generating unit and the magnetic flux focusing unit. However, the device is only suitable for detecting the magnetic flux but not suitable for detecting the AC magnetic susceptibility and the phase difference of the magnetic field.
U.S. Pub. NO. 20090102458 provides a device for measuring magnetic susceptibility. Although the device is able to measure the magnetic susceptibility of a magnetic particle when the magnetic particle is within a magnetic field and interacting with the magnetic field, but the device can not measure the phase difference of the magnetic field and the real part and the imaginary part of the AC magnetic susceptibility. Comparing with the device described in U.S. Pub. NO. 20090102458, the device of the present invention can not only measure the magnetic susceptibility intensity and phase difference of under testing particles, but also extract the real part and the imaginary part of the AC magnetic susceptibility and the changes of the AC magnetic susceptibility of the under testing particles in various frequencies.
U.S. Pat. No. 7,541,805 provides a system with superconducting quantum interference device for measuring magnetic characteristics. The superconducting quantum interference device detects the magnetic flux variation of a pickup coil and outputs an AC signal when under testing particles are disposed within an AC magnetic field. The AC signal includes a real part signal and an imaginary part signal. The real part signal and the imaginary part signal may be separated inaccuracy when the AC signal does not equal to a reference signal. Therefore, a phase regulation for the measuring result is needed for reducing the inaccuracy. The AC signal of the system includes a real part signal and an imaginary part signal for determining if the phase regulation needed to be executed for reducing the phase difference and making the measuring result more accurate. However, the system can not detect the changes of magnetic susceptibility intensity and phase difference in various frequencies and extract the real part and the imaginary part of the AC magnetic susceptibility. Moreover, the system with superconducting quantum interference device described in U.S. Pat. No. 7,541,805 is suitable for precision instrument department due to its high-priced and tightly operation conditions. For industrial circles without precision instrument department or basic purposes in general academic circles speaking, the system is unsuitable. Comparing with the system with superconducting quantum interference device described in U.S. Pat. No. 7,541,805, the device of the present invention can not only measure the magnetic susceptibility intensity and phase difference of under testing particles, but also extract the real part and the imaginary part of the AC magnetic susceptibility and the changes of the AC magnetic susceptibility of the under testing particles in various frequencies.
U.S. Pat. No. 6,825,655 provides a method for detecting changes of magnetic response of at least one magnetic particle provided with an external layer in a carrier fluid. The method comprising measuring the characteristic rotation time of the magnetic particle with respect to the external layer, and measuring Brownian relaxation in the carrier fluid under the influence of an external alternating magnetic field. The method implies that upon modification of the effective volume of the particle or its interaction with the carrier fluid, a hydrodynamic volume of the particle changes, which implies a change of the frequency (fmax) at which an out of phase component of the magnetic susceptibility has its maximum. However, problems with this technique are that it is not sensitive as compared with other presently available sensors and high-priced.
The present invention utilizes an AC magnetic susceptibility coil set with simply construction for overcoming the problems (can not measure the phase difference of the magnetic field, the real part and the imaginary part of the AC magnetic susceptibility and the changes of the AC magnetic susceptibility of the under testing particles in various frequencies) aforementioned in U.S. Pat. No. 6,683,452 and U.S. Pub. NO. 20090102458. Moreover, the present invention also overcomes the problems of high-priced, tightly operation conditions, only suitable for precision instrument department and not sensitive aforementioned in U.S. Pat. Nos. 7,541,805 and 6,825,655.